[GitHub] [arrow] edponce commented on pull request #13009: ARROW-602: [C++] Provide iterator access to primitive elements inside an Array

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edponce commented on PR #13009:
URL: https://github.com/apache/arrow/pull/13009#issuecomment-1112256516

   Here are some observations on the `Array/ChunkedArray` iterator support in Arrow.
   
   1. For `ChunkedArray`, how would we know if the iterator is in the begin/end?
       I suggest we add begin/end methods to `ChunkedArray` ( [for example, see `NumericArray`](https://github.com/apache/arrow/blob/master/cpp/src/arrow/array/array_primitive.h#L118-L120)) so that it can be traversed in for-loop idioms.
       Note: Currently, not all Arrow `Arrays` have begin/end methods.
       ```c++
       # Iterator
       for (auto it = chunked_array.begin(); it != chunked_array.end(); ++it) { ... }
   
       # STL for_each
       std::for_each(chunked_array.begin(), chunked_array.end(), [](T const& elem) { ... }
   
       # range-for
       for (const auto& elem: chunked_array) { ... }
       ```
   
   2. C++ has begin/end and their const variants cbegin/cend. In Arrow `Array` and `ChunkedArray` are immutable types, so I would expect for them to only support cbegin/cend, but range-for requires begin/end. A solution is to have begin/end return a const iterator.
   
   3. There is an [experimental `MultipleChunkIterator`](https://github.com/apache/arrow/blob/master/cpp/src/arrow/chunked_array.h#L194) that seems to not be used in the codebase. Should it be removed?


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